The invention relates to a method of producing a highly oriented yarn (HOY) from a thermoplastic material and a spinning apparatus for melt spinning a highly oriented yarn.
In the production of synthetic multifilament yarns from a thermoplastic melt in one process step, one distinguishes basically between partially drawn and fully drawn yarns. The partially drawn yarns, which are also described as preoriented yarns (POY), have a partially oriented molecular structure that requires a subsequent drawing in a second process step. In comparison therewith, fully drawn yarns (FDY) are suited for direct further processing without a subsequent drawing. The FDY yarns are drawn in the spinning process at a high ratio by means of draw systems, so that an aligned molecular structure is achieved in the polymer.
To produce a yarn with a highest possible orientation of the molecules of the polymer, methods are also known wherein the yarn is drawn at a high ratio while firming up directly before the solidification of the polymer. In these yarns, known as highly oriented yarns (HOY), a stress-induced crystallization leads to the orientation of the molecules in the polymer. In comparison with the FDY yarns, the known HOY yarns have a lower elastic limit. Depending on the method of further processing, this can lead, due to the force acting upon these yarns, to a permanent deformation and, thus, to an irregular dyeability. The known HOY yarns are totally unsuitable for methods of further processing, wherein major stress peaks act upon these yarns.
Although it is theoretically possible to increase the elastic limit of HOY yarns by increasing the withdrawal speed, there are physical limits set to this process, since in the melt spinning of HOY yarns, the filaments forming the yarn may have only a limited crystallinity during drawing to ensure a safe withdrawal without damage to the yarn. A too highly precrystallized filament is much too frozen in its structure to withstand, without being overstressed, the forces developing in the yield point.
EP 0 530 652 and U.S. Pat. No. 5,612,063, disclose an apparatus and a method for producing a synthetic yarn, wherein the filaments undergo a delayed cooling before their solidification. This further delays crystallization of the filaments, which leads to an increased elastic limit of the yarns. However, the known apparatus and method have the disadvantage that the length of the delayed cooling can be only very limited, since lacking stabilization of the filaments by the air flow represents within this region an increasing risk that the filaments stick together.
EP 244 217, and U.S. Pat. Nos. 5,141,700 and 5,034,182 propose to remove the filaments after passing through a pressurized cooling shaft from the cooling shaft by means of an air stream. This also achieves a delayed crystallization of the filaments. Likewise in EP 0 682 720, a delayed crystallization of the polymer is realized, in that an accompanying air stream is directed onto the filaments before solidification.
The apparatus and methods known in the state of the art are all aimed at producing a synthetic yarn at highest possible takeup speeds without its physical properties undergoing a substantial change. Thus, in these known methods, the decrease in elongation at higher withdrawal speeds is compensated by the delayed crystallization of the polymer in the spinning line. However, these methods are unsuitable for producing HOY yarns with higher elastic limits and with higher tenacities.
In the production of a highly oriented yarn, there exists the problem that the known yarns have too high elongation values and too low tenacity values. The elongation values of the yarn may be improved by increasing the withdrawal speed. An increase in the withdrawal speed, for example, in the apparatus disclosed in EP 0 530 652 and U.S. Pat. No. 5,612,063, is bound to lead to an increase in the withdrawal tension, which results, however, in that the filaments are overstressed during the drawing due to the low tenacity of the filaments.
It is an object of the invention to provide a method and a spinning apparatus for producing a highly oriented yarn (HOY), which exhibits elongation and tenacity values typical of a fully drawn yarn (FDY), and which can be produced with a high spinning reliability.
The above and other objects and advantages of the present invention are achieved by the provision of a method and apparatus wherein the melted thermoplastic material is extruded through a nozzle of a spinneret to form a plurality of downwardly advancing filaments, and such that the filaments solidify at a location spaced below the nozzle. The filaments are withdrawn from the nozzle under a withdrawal tension so as to cause the filaments to be drawn while being solidified, with the withdrawal tension being generated by a withdrawal speed of at least about 6500 m/min. In addition, the filaments are assisted in their advance before their solidification such that before their solidification the filaments are relieved from tensile stress and during solidification and drawing a reduced withdrawal tension is effective on the filaments. The filaments are also combined during their advance after their solidification, to form an advancing multifilament yarn, which is then wound into a package.
The invention is based on the recognition that overstressing of the filaments can occur in the process of the yarn formation. In high speed spinning, one observes no uniform rise of the yarn speed between the yarn exit from the spinneret and the solidification point of the filaments. After the filaments emerge from the spinneret, a relatively slow acceleration sets in initially, until the start of the stress-induced crystallization. Within few centimeters, the stress-induced crystallization leads to an acceleration of the filaments to the withdrawal speed. In this instance, the tenacity of the filaments must be greater than the forces necessary for the acceleration of the yarn, to avoid filament breakage. In accordance with the invention, the filaments are assisted in their advance before they solidify such that no significant additional tensile stresses resulting from frictional forces of the air act upon the filaments before they solidify. Thus, the filaments are relieved before their solidification, so that a reduced withdrawal tension is effective on the filaments while being drawn during their solidification. With that, one realizes on the one hand a high orientation of the molecules during drawing. On the other hand, a high withdrawal speed is made possible with a correspondingly high withdrawal tension. In this process, the withdrawal tension is generated by a withdrawal speed of at least 6,500 m/min. It has shown that it is thus possible to produce a highly oriented yarn with tenacity values greater than 4cN/dtex and an elongation in the range of 30%.
To assist the movement of the filaments before their solidification or to bring about a relief of the forces engaging on the filaments before their solidification, it is possible to apply basically two variants of the method according to the invention. In a first variant, the speed of the advancing yarns is increased before drawing by a higher injection speed in the extrusion of the filaments. In practice, this possibility can be used only to a certain extent due to the high pressure drops upstream of the spinneret.
In the second variant of the method, the air friction acting upon the filaments is influenced. To this end, the filaments advance after their extrusion through a cooling medium. Directly before the solidification of the filaments, a cooling medium stream is generated that assists the movement of the filaments. This measure effects a reduction of the air friction that exerts a braking effect on the filaments. The cooling medium in use is preferably air.
In a particularly advantageous embodiment of the method, the cooling medium stream has a flow velocity that is substantially the same as the speed of the advancing filaments before their solidification. Thus, no braking flow forces are operative on the filaments, so that the advancing speed of the filaments further increases.
For a further reduction of the tensile forces that are operative during the solidification, it is possible to generate the cooling medium stream with a flow velocity that is greater than the speed of the advancing filaments before they solidify. This permits production of highly oriented yarns of a great tenacity at even higher process speeds.
In a preferred embodiment of the method, for purposes of generating the cooling medium stream, the filaments advance through a constrictor and a diffuser. This allows to generate the cooling medium stream purposefully at one point over a very short distance of the spinning line. Preferably, the narrowest cross section of the constrictor is placed in the spinning line such that it is shortly before the solidification point of the filaments. This measure permits reducing a stress-induced preorientation within the filaments. The yarn firms up within a very short distance, which leads to a particularly high orientation of the molecule chains in the polymer.
In a particularly advantageous further development of the method, the filaments advance after their extrusion and before their solidification through a cooling shaft that connects to ambient air through an air-permeable cylindrical wall. Thus, a delayed cooling of the filaments is realized, so that the yield forces are advantageously influenced and lead to a further relief of the withdrawal tension. This measure is advantageous in two respects, since it permits on the one hand an increased withdrawal tension during the drawing of the filaments, and since on the other hand the delayed cooling substantially reduces a preorientation of the still molten filaments.
This measure can be still further improved by a variant wherein the filaments advance directly after emerging from the spinneret through a heating zone, wherein an amount of heat is supplied to the filaments.
To operate the method with the least possible expenditure for apparatus, the withdrawal tension may be generated directly by the winding speed of a takeup device.
To produce, if possible, a qualitatively superior and uniform yarn, it is desirable to use a variant of the method wherein the withdrawal tension is defined by a feed system. The feed system is arranged upstream of the takeup device, so that fluctuations in the yarn tension due to the winding can advantageously not become effective in the spinning line. It is possible to produce the yarn with a very uniform withdrawal tension.
In accordance with the invention, it becomes possible to produce a highly oriented yarn with substantially similar properties to a fully drawn yarn by influencing the spinning line. In this connection, the spinning apparatus of the present invention has been found especially advantageous for carrying out the method. In accordance with the invention, a constrictor and a diffuser arranged on the outlet side of the constrictor form a cooling device. The constrictor effects a great acceleration of the air entrained by the filaments. In this process, the cooling air stream is accelerated to a maximum speed in the narrowest cross section. Directly after passing the narrowest cross section of the constrictor, the diffuser causes the cooling air to expand. Thus, the flow velocity of the air slows down, thereby assisting the filament movement for a very short time. A longer treatment zone that favors a preorientation is avoided.
A cooling cylinder composed of an air permeable tubular wall may be positioned between the spinneret nozzle and the constrictor. This helps ensure that no air turbulences develop that influence the advance of the filaments as they enter the constrictor.
In the variants of the method, wherein it suffices to reduce or avoid air frictions that slow down the advance of the filaments for producing a highly oriented yarn, it is preferred to construct the spinning apparatus with the diffuser connected to a vacuum generator.
In this connection, it is possible to avoid turbulence at the outlet end of the cooling device during the expansion of the air stream surrounding the filaments, by constructing the spinning apparatus so that the diffuser connects at its outlet end to an air permeable tubular screen cylinder and which is part of a vacuum chamber which is connected to the vacuum generator. Thus, entrained air is uniformly removed over the entire circumference of the filament bundle.
To realize in the production of the yarn a favorable flow profile, it has been found that the constrictor should have in its narrowest cross section a diameter from at least 10 mm to at most 40 mm.
To make available an adequate quantity of air in the spinning line and in particular in the center of the filament bundle for building up the air stream as well as for cooling the filaments, the cooling cylinder may be subdivided in the direction of the advancing yarn into several zones, with each zone having a wall with a different gas permeability. This configuration makes it possible to influence the quantity of air flowing into the cooling shaft irrespective of the filament speed and irrespective of the differential pressure between the cooling shaft and the surroundings. Thus, it is possible to exert a purposeful influence on the properties of the filaments. The quantity of air entering through the wall of the inlet cylinder is in this instance proportionally dependent on the gas permeability or porosity of the wall. Accordingly, in the case of a high permeability to gas, a larger quantity of air per unit time is admitted into the cooling shaft under otherwise constant conditions. Conversely, in the case of a low permeability to air of the wall a proportionately smaller quantity of air enters the spin shaft. The transition of the pas permeability from the one zone to the other is made preferably stepless to avoid greater differential flows. However, a stepped transition of the gas permeability is likewise possible.
In the production of the yarn, it is especially important that each filament in the spinning line be evenly treated until their combination into a yarn. The nozzle bores of the spinneret are preferably arranged in one or more annular lines of bores, with the bores of each line being equally spaced apart. This ensures that the flow generated in the constrictor is uniformly effective on each of the filaments.
In a further development of the spinning apparatus according to the invention, the yarn is withdrawn from the spinneret by means of a feed system. This allows to adjust the withdrawal tension and yarn tension independently of each other when the yarn is wound. Furthermore, it is possible to generate a highly uniform withdrawal tension.
To be able to produce in a spinning plant a plurality of parallel side by side yarns, the feed system preferably comprises two rolls which are partially looped by the advancing yarn, and with at least one of the rolls being driven. In this embodiment, a decrease in the yarn tension may be adjusted by means of the amount of looping by the yarn on the rolls.
To prevent a premature preorientation of the filaments, a heating device may be provided between the nozzle of the spinneret and the cooling cylinder for thermally treating the filaments.
Both the method and the apparatus of the present invention are suitable for producing highly oriented textile yarns of polyester, polyamide, or polypropylene.